Interestingly, we discovered some very useful tools in the perspective
of linear Partial differential equations/systems theory.
Even though, in this article, we only consider the systems with
coefficients independent of \(x\) , the sharp function estimates and
the tools used to derive them will naturally lead to many
subsequent works studying, for instance, elliptic and
parabolic equations and systems with discontinuous coefficients defined in an arbitrary domain \(U\)  of \(\mathbb {R}^d\) . In this context, we refer the readers to very extensive literature [1]} and recent articles [2]}, [3]}, [4]}, [5]}, [6]} (also see the references therein),
where (standard) \(L_p\) -theories are constructed for single equations with VMO (or small BMO)-coefficients.
